Polycrystalline ceramic is generally opaque since a light is scattered due to pores, grain boundaries, or impurities. However, when such causes for light scattering are removed, the polycrystalline ceramic may become transparent like a single crystal. According to light-transmissive alumina that is best-known, alumina is capable of transmitting a light by removing most pores by performing atmosphere-sintering using high purity powders and removing grain boundaries by increasing sizes of crystal grains. However, since crystals have anisotropic hexagonal shapes, a light transmittance is affected by directions of the crystal grains, and thus the alumina becomes light-transmissive, instead of transparent like glass.
Aluminum oxynitride (Al23O27N5) is commonly denoted by “AlON”, and in detail, γ-AlON has an isotropic cubic shape and is a distinctive material that may become transparent since pores are relatively easily removed due to satisfactory sinterability. However, when AlON is applied to a high intensity wear-resistant transparent window instead of transparent tempered glass or sapphire, the value of AlON may increase when AlON has very a high light transmittance close to about 85% that is a light transmittance of sapphire. Accordingly, pores of AlON ceramic, which are biggest causes for reducing a transmittance, are removed as much as possible, thereby easing the reduction of the transmittance caused by a thick thickness. In relation to such a material, U.S. Pat. No. 4,241,000 first disclosed a method for preparing polycrystalline AlON having a light transmittance by mixing Al2O3 powder and AlN powder, thermally treating the powder mixture under a nitrogen gas atmosphere for 24 hours at 1200° C., and then sintering the thermally treated powder mixture at 1800° C. under an atmospheric pressure.
Also, according to U.S. Pat. No. 4,520,116, a polycrystalline whose relative density is equal to or higher than 99% and a visible-light transmittance of a specimen having a thickness of 1.78 mm is 43% is prepared by calcining AlN and Al2O3 powders to synthesize AlON powder, and then adding a small amount of a boron (B) compound, an yttrium (Y) compound, or a lanthanum (La) compound as a sintering aid to the AlON powder.
Also, U.S. Pat. No. 4,481,300 and U.S. Pat. No. 4,686,070 disclose a method for preparing AlON having an infrared-light transmittance of 80% at a thickness of 1.45 mm by mixing Al2O3 powder and carbon black powder in a suitable ratio and calcining the mixture at a temperature around 1600° C. to prepare Al2O3 and AlN, thermally treating Al2O3 and AlN at a temperature around 1800° C. in a boron nitride (BN) container to synthesize AlON, ball-milling AlON for a long period of time to prepare minute AlON powder, molding the minute AlON powder, and then sintering the molded AlON powder under a nitrogen gas atmosphere and atmospheric pressure for 24 to 48 hours at a temperature from 1900° C. to 2140° C., wherein a Y compound and a La compound is added as a sintering accelerator.
Also, U.S. Pat. No. 4,720,362 discloses a preparing process of adding about 0.5 wt % of a B compound or a Y compound to AlON powder, molding the AlON powder, and then sintering the molded AlON powder for 20 to 100 hours at a temperature equal to or higher than 1900° C. Here, B2O3 that is the B compound or Y2O3 that is the Y compound, which is added as a sintering additive, forms a liquid phase during sintering to accelerate densification during the beginning and middle of the sintering and to prevent growth of abnormal crystal grains as a secondary phase solute-dragged or precipitated pins grain boundaries during the end of the sintering. Accordingly, pores are prevented from being no longer removed as the pores enter crystal grains.
Also, U.S. Pat. No. 5,231,062 discloses a method for preparing transparent aluminum magnesium oxynitride (AlMgON) by adding 2.0 to 16 wtT of MgO.
Also, U.S. Pat. No. 5,688,730 discloses a method for preparing AlON powder by mixing Al2O3 and AlN powders having a relatively high specific surface area.
Also, U.S. Pat. No. 6,955,798 discloses a method for preparing AlON powder by thermally treating a mixture of Al and AlO powders under a nitrogen gas atmosphere to prepare a nitrated mixture of Al and AlO, milling the nitrated mixture, and then re-thermally treating the milled nitrated mixture at a sufficiently high temperature.
Also, U.S. Pat. No. 7,045,091 discloses a method for preparing transparent AlON, wherein, instead of first synthesizing AlON powder and then sintering the AlON powder generally performed to prepare transparent AlON, a powder mixture of Al2O3 and AlN is sintered with a help of a liquid phase at a temperature from 1950° C. to 2025° C. in which the liquid phase and a solid phase coexist, and then the powder mixture is re-sintered to change the liquid phase to the solid phase at a temperature that is lower than 1950° C. to 2025° C. by at least 50° C., in which only the solid phase exists. However, a visible-light transmittance of the transparent AlON prepared as such and having a thickness of 1 mm only exceeds 10%.
Also, U.S. Pat. No. 7,163,656 discloses a method for preparing high density AlON regardless of transparency, via uniaxial hot pressing. The uniaxial hot pressing is used to obtain theoretical high density or to sinter a material that is difficult to be densified due to low sinterability. However, since the uniaxial hot pressing is uniaxial pressing, a shape after sintering is largely limited, productivity is low, and costs are high. Also, since a graphite mold is used for pressing, a color of the AlON becomes generally black, and thus it is difficult to prepare a transparent product.
AlON starts to evaporate at a temperature equal to or higher than 1950° C. Evaporation during high temperature sintering may be reduced if possible, and AlON is easily suppressed from being evaporated even at a low nitrogen gas pressure from 0.1 MPa to 0.3 MPa. Here, an overpressure of 1 atm to 3 atm is applied to a nitrogen gas via electricity during a general atmospheric pressure sintering under a flowing nitrogen gas atmosphere, and thus costs are barely increased. Alternatively, a special pressure electric furnace may be used to remarkably increase a gas pressure up to about 10 MPa, and a gas pressure sintering (GPS) furnace may be used during GPS, but costs are increased and productivity is decreased. The GPS is developed to further increase sintering density by suppressing silicon nitride from evaporating during high temperature sintering and increasing sintering power by using a gas pressure, and may be used to prepare AlON, but product sizes are limited and costs are remarkably increased. Furthermore, hot isostatic pressing (HIP) for applying a gas pressure around 200 MPa may be used, but a size of a high pressure chamber is further decreased and costs are further increased.
Since most AlON ceramics prepared via such general technologies above are prepared by mixing a suitable amount of carbon powder with Al2O3 or by thermally treating AlN and Al2O3 powders at a high temperature to separately synthesize AlON powder, costs are increased. Also, since sintering is performed at a temperature equal to or higher than 2000° C., or is performed at a temperature lower than 2000° C. for a long period of time in order to increase transparency, costs are further increased. On the other hand, it is difficult for AlON ceramic that is prepared by sintering Al2O3 and AlN powders under an atmospheric pressure to have high transparency.
Meanwhile, WO 2008-047955 filed by the present inventors discloses a method for preparing AlON by sintering Al2O3 and AlN powders, wherein 0.1 to 0.2 wt % of MgO is added as a sintering aid, as well as a well-known sintering aid, under a condition where the content of AlN is fixed to 35 mol %, and transparent AlON whose visible-light transmittance reaches 80% in a specimen having a thickness of 1.9 mm is prepared by performing presintering at a temperature of 1650° C. and then performing final sintering. However, here, the final sintering is performed for 5 hours at a relatively high temperature of 2000° C., and despite of such final sintering, the visible-light transmittance of the specimen having the thickness of 1.9 mm does not exceed 80%. Accordingly, a new method for preparing AlON having a largely enhanced transmittance despite of a low final sintering temperature from 1950° C. to 1970° C. is required.
A chemical formula of AlON is generally Al23O27N5, or may be Al(64+x)/3O(32-x)Nx based on nonstoichiometric determination capable of having a relatively large range of N, wherein AlON is in a single phase even when the number of N is higher than or smaller than 5.
However, in the above general technologies, a visible-light transmittance according to a ratio of Al2O3 and AlN powders, i.e., an effect of an “x” value in Al(64+x)/3O(32-x)Nx on a visible-light transmittance, is not studied with respect to preparing transparent AlON by sintering AlON powder after synthesizing the AlON powder as a separate process or by sintering a powder mixture of Al2O3 and AlN powders.
The general technologies assume that 27 to 40 mol %, i.e., the “x” value of 3.4 to 6.0, is suitable, and in all academic researches, AlON is prepared or studied by fixing the content of AlN to 35.7 mol %, i.e, the “x” value to 5.0, in the general chemical formula of Al23O27N5, or fixing the content of AlN to 30 mol %, i.e., the “x” value to 3.9.
Also, according to the paper “AlON: A Brief History of Its Emergence and Evolution” presented in Journal of the European Ceramic Society 29 (2009), the “x” value of AlON ceramic is 4.0, i.e., AlN is 31 mol %, in Surmet Corporation, U.S., that solely successfully commercialized transparent AlON worldwide. According to the paper, AlON powder is synthesized and sintered, and in order to obtain high transparency, AlON is prepared to have an average size of crystal grains from 200 to 250 μm. In order to obtain such a large size of crystal grains, sintering may be performed for a long time at a relatively high sintering temperature. Generally, crystal grains having a large size cause deterioration of strength of ceramic.
Accordingly, the present inventors suitably adjusted a molar fraction of Al2O3 and AlN, which are raw-material powders, while preparing AlON prepared via sintering, so as to increase sinterability, thereby further enhancing a light transmittance of AlON. In detail, generally, when sinterability of ceramic is increased, the same density may be obtained in a short time at a lower final sintering temperature.